Only a handful of pages appearing in a site: search

[MattVTS]

As a reasonably experienced SEO I haven't come across this before. The site I'm working on has almost zero visibility for even very localised niche terms.

The site in question is Versantus (sorry can't post a link yet, but if you search for the specific brand name you'll find it)
The site sits on www, and carrying out a search of site:<domain> returns, at best, 5 spurious URLs from the site, and frequently just returns the home page.

This is a Drupal web design agency based in Oxford UK.
There are only 2 or 3 other Drupal agencies in the area so I would expect a specific local search such as Drupal agency Oxford to return at least the home page. Instead the site isn't appearing in the top 100 results.

The site has good domain authority, and scores 97% in an SEMRush site audit. Content is optimized and I can't see any manual penalties against it. We have a good disavow list set up, sitemap.xml in good order (I think).

The site is sluggish on mobile, but not enough to cause what seems like a total lack of indexing in Google.
What else might be causing this?